Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information reaches the cochlea and the hair cells. Sensorineural hearing loss, on the other hand, is usually due to the absence or impairment of the hair cells which are needed to transduce acoustic signals in the cochlea into nerve impulses that are sent to the auditory nerve. People suffering from severe sensorineural hearing loss are usually unable to derive any benefit from conventional hearing aid systems because their mechanisms for transducing sound energy into auditory nerve impulses are non-existent or have been severely damaged.
Cochlear implant technology seeks to overcome sensorineural hearing loss by bypassing the hair cells in the cochlea and presenting electrical stimulation to the auditory nerve directly, leading to the perception of sound in the brain and at least partial restoration of hearing. Indeed, cochlear implant technology may be used to bypass the outer, middle and inner ears. Cochlear implant systems that utilize such technology have been successfully used to restore hearing in sensorineural deaf patients.
Generally, a cochlear implant system includes an external portion and an implanted portion that are separated by a skin barrier. The external portion usually includes a power source, a microphone and a signal processing device, whereas the implanted portion usually includes a stimulation device and an electrode array. The power source supplies power to the system. Sound enters the system through the microphone which delivers it to the signal processing device as an electrical signal. The signal processing device processes the signal and transmits it to the stimulation device through the skin barrier. The stimulation device uses the received signal to stimulate electrodes in the electrode array that is implanted into the cochlea. The electrodes in the array transmit electrical stimuli to the nerve cells that emanate from the cochlea and that are part of the auditory nerve. These nerve cells are arranged in an orderly tonotopic sequence, from high frequencies near the initial (basal) end of the cochlear coil to progressively lower frequencies towards the inner end of the coil (apex). Nerve cells emanating from the various regions of the cochlea are associated with the frequencies that most efficiently stimulate those regions, and the brain, which receives neural impulses from the auditory nerve, maps those frequencies in accord with this association.
Conventional cochlear implants separate sound signals into a number of parallel channels of information, each representing the intensity of a narrow band of frequencies within the acoustic spectrum. Ideally, each channel of information would be conveyed selectively to the subset of nerve cells located along the cochlea that would have normally transmitted information about that frequency band to the brain. This would require placing the electrode array along the entire length of the cochlear ducts, which is surgically impractical. Instead, the electrode array is typically inserted into the scala tympani, one of the three parallel ducts that make up the spiral shape of the cochlea. The array of linearly arranged electrodes is inserted such that the electrode closest to the basal end of the coil is associated with the highest frequency band and the electrode closest to apex is associated with the lowest frequency band. Each location along the implanted length of the cochlea may be mapped to a corresponding frequency, thereby yielding a frequency-to-location table for the electrode array. The foregoing illustrates the relationship between frequency and physical location in the cochlea—i.e., the cochlear frequency/location correspondence.
The performance of a cochlear implant system is limited mainly by the amount of information that can be delivered by electrical stimulation to the patient, which, in turn, is limited by the number of electrodes in the implant. The number of electrodes that can be used is limited by the size of the scala tympani and the distance or spatial separation between electrodes. While the size of the scala tympani presents an anatomical limitation, it is possible to reduce the distance between electrodes. However, reducing such spatial separation increases electrode interaction and interference, which could have undesirable effects. It is however possible to use such effects to deliver additional spectral information in a suitable manner.
Recent studies have shown that simultaneously stimulating two adjacent electrodes in such systems results in patients perceiving a pitch that is between the two pitches perceived when each electrode is stimulated individually. Moreover, as the stimulation current is changed from being entirely applied to the first electrode to the second electrode, pitch sensation changes from the pitch associated with the first electrode to the pitch associated with the second electrode in a continuous fashion. This is because the electric field resulting from stimulating one of the electrodes is likely to be superposed to that resulting from stimulating the other electrode. The superposed electric fields are centered around a virtual electrode that lies between the two adjacent electrodes. Furthermore, the perceived loudness stays roughly constant so long as the sum of the currents applied to the electrode pair stays roughly constant. Thus, it is possible to stimulate a virtual electrode located between adjacent electrodes by simultaneously stimulating these electrodes using relative current weights, whereby the frequency band associated with the virtual electrode corresponds to one that lies between the frequency bands associated with the individual electrodes.
The challenge lies in utilizing a sound processing strategy that makes use of such virtual electrodes. Although separate channels could be assigned to individual electrodes and virtual electrodes, such a processing scheme limits the number of virtual electrodes that may be stimulated unless the number of channels is significantly increased. Increasing the number of channels, however, demands additional processing capabilities which may result in delays or may require more power and more complicated circuitry.
Frequencies that may be associated with optimal virtual electrodes may instead be estimated. One example of a known system that estimates frequencies using conventional methods, such as calculating the instantaneous frequency along with phase angle and magnitude values, is described in U.S. Pat. No. 6,480,820. However, such methods result in inconsistent representations of spectral sound information. This is because adding sound components to the sound stimulus may result in a drastic and disproportional shift in the location on the electrode array to be stimulated. Thus, there is a need for improved frequency computation for presentation of spectral information in sound processing strategies that can be used with cochlear implant systems that utilize simultaneous stimulation of several electrodes.
When a cochlear implant is provided to a patient, it is necessary to initially fit the system in order for it to better perform its intended function of helping a patient to sense sound at appropriate levels. A common method of fitting involves presenting a known sound stimulus to a patient while a subset of the electrode array is activated and adjusting the level of corresponding electrical current applied to the array such that the sound perceived by the patient is of appropriate loudness. In applying such a method, it is assumed that the perceived loudness will not be affected by the activity of adjacent electrodes once the full array is activated, including electrodes that were previously deactivated. However, when adjacent electrodes are stimulated simultaneously, the resulting electric fields are likely to be superposed thereby affecting the loudness perceived by the patient, as mentioned above. Thus, there is a need for fitting sound processing strategies used with cochlear implant systems that utilize simultaneous stimulation of several electrodes.
In addition to accounting for spectral information and loudness, a cochlear implant system should preserve, as much as possible, temporal information that is key to differentiate various sounds. Presenting fine temporal information is critical for the perception of overall sound quality, clarity, speech and music. There is therefore also a need for improved time detection for presentation of temporal information in sound processing strategies used with the cochlear implant systems described above.